Refrigeration systems are commonly used for cooling a desired area. Refrigeration works by removing heat from an enclosed area and transferring that heat to an external atmosphere located outside of the enclosed area. Refrigeration systems are widely used in refrigerators, air-conditioning units in homes and automobiles, and cargo areas of ships and trucks.
FIG. 1 illustrates a block diagram of a basic refrigeration system 100. The refrigeration system 100 includes a compressor 102, a condenser coil 104, a condenser fan 106 with a condenser motor 108, an expansion valve 110, an evaporator coil 112, an evaporator fan 114 with an evaporator motor 116, and refrigerant 118. Refrigerant is a fluid used to absorb and transfer heat. Refrigerant absorbs heat by evaporating from a liquid to a gas at a low temperature and pressure. Refrigerant releases heat by condensing from gas back to liquid at a higher temperature and pressure.
Refrigerant 118 enters the compressor 102 in a low-temperature, low-pressure gas state. The compressor 102 compresses the refrigerant 118 to a high-temperature, high-pressure gas state. The refrigerant 118 then flows through the condenser coil 104, wherein the refrigerant 118 releases heat until liquefied. Heat in the refrigerant 118 is rejected by the condenser coil 104. The condenser fan 106 circulates ambient air across the condenser coil 104, transferring heat from the condenser coil 104 to the external atmosphere. The expansion valve 110 then reduces the pressure of the refrigerant 118 as the refrigerant 118 flows through the expansion valve 110, creating a low-temperature, low-pressure mixture of liquid and vapor refrigerant. The low-temperature, low-pressure refrigerant mixture 118 then flows through the evaporator coil 112. The evaporator fan 114 draws warm air from a desired area to be cooled 120 across the evaporator coil 112 carrying the cold refrigerant mixture 118. Heat is then absorbed by the refrigerant 118 as it flows through the evaporator coil 112. As the refrigerant 118 absorbs the heat, the refrigerant 118 changes phase from liquid back to gas. The cycle then repeats.
In order for the refrigerant 118 to absorb and reject the maximum amount of heat, the components in the refrigeration system 100 should operate efficiently if the compressor 102, the condenser motor 108, and the evaporator motor 116 are all driven by three-phase motors. Three-phase motors are widely used because they are efficient, economical, and durable. Three-phase motors work by introducing three electrical phases through terminals, each of the phases energize an individual terminal and reach a maximum at different times within a cycle.
In FIG. 2, a three-phase motor 202 has three terminals labeled as A, B, and C. Terminals are energized, wherein terminal B is lagging 120° behind terminal A, and terminal C is lagging 120° behind terminal B and 240° behind terminal A. Thus, each of the terminals A, B, and C are energized 120° apart from each other. The terminals A, B, C are energized by a three-phase power supply 204, which also has three terminals A′, B′, and C′. The terminals A′, B′, C′ of the three-phase power supply 204 determine the phase rotation of the terminals. The phase rotation determines the rotation of the three-phase motor 202. For example, if terminals A′, B′, C′ of the three-phase power supply 204 are wired to the terminals A, B, C of the three-phase motor 202, respectively, and have a phase rotation of A′, B′, and C′, then terminals A, B, C have the same phase rotation and the three-phase motor 202 rotates clockwise. In clockwise rotation, terminal C lags 240° behind terminal A.
However, if the terminals A, B, C of the three-phase motor 202 are improperly wired to the terminals A′, B′, C′ of the three-phase power supply 204, then the three-phase motor 202 will rotate counter-clockwise (i.e. “in reverse”). For instance, if terminals B and C are swapped wherein terminal B is wired to terminal C′ and terminal C is wired to B′, then energizing terminals A′, B′, C′ will result in terminals A, C, B being energized. Swapping two terminals will result in a 120° phase shift, i.e. C would lag 120° behind A. A phase shift of 120° will cause the three-phase motor 202 to rotate in reverse.
Reverse rotation of three-phase motors will reduce efficiency and may even cause damage to some of the components driven by these motors in refrigeration systems. For example, a scroll compressor may be damaged if operated in reverse rotation. By contrast, a reciprocating compressor may run in either direction without being effected if equipped with a reversible oil pump. If a condenser motor or an evaporator motor operates in reverse, then condenser fan or evaporator fan efficiency will decrease and impact heat transfer efficiency of the heat exchanger. For instance, if the evaporator fan operates inefficiently, then adequate heat will not be drawn from the area to be cooled, and the refrigeration unit will have lower cooling capacity. While the refrigeration system will still operate when the condenser motor operates in reverse, the heat rejection capacity of the condenser coil will decrease. Therefore, depending on the components that these motors drive, each of the three-phase motors in a refrigeration system are impacted by improper wiring differently. In a single system with multiple three-phase motors, it is important to determine the priority of operation for each of these three-phase motors.